Virtue is the sole good. If you want a detailed of explanation about this, see this Reddit post. You can also read the entire chapter called Joy and Virtue in The Inner Citadel by Hadot.
In this passage, Marcus quite simply delineates what truly is good. He lists out the four main virtues (wisdom, discipline, justice and courage). Then he applies a popular saying to both these virtues and to indifferents.
Applied to wealth or health:
- she is so rich, she has no room for anything else.
- he is so strong and has so many muscles, he can't even walk normal.
Applied to courage or self-control:
- he has so much courage, he has no place to sit??
- she has so much self-control, she can't even walk??
When applied to indifferents, indeed you can have too much. But when applied to a virtue, it's nonsensical.